It is well known to provide illumination of instrument pointers by incorporating a small lamp on a pointer or by using light conducting transparent pointer material optically coupled to a near-by light source. In the case of the small lamp on the pointer two problems arise: the expense of providing a lamp which is sufficiently small and sufficiently bright, and the reliability of the electrical leads to the lamp which must be flexible to allow free pointer motion.
In the case of light conduction schemes for pointer illumination it is the usual practice to transmit light from a remote lamp through more than one transparent element and into the pointer in such a manner that many reflections are required and that a very small portion of the source light enters the pointer, resulting in a low level of pointer illumination. It is also customary to couple light to a pointer, whether directly from a lamp or via a light conductor, through an off-axis surface of the pointer so that the light enters the pointer unsymmetrically and the pointer illumination then varies as the angular position changes. In many cases, the pointer illumination restricts the pointer movement to a small range.
It is desirable for a pleasing and useful display that the pointer illumination be sufficiently bright and that the brightness should not change during the pointer movement. In some applications a display is covered by a dark lens so that pointers are concealed from view when not in use and are revealed when illuminated. There a bright pointer is required to show through the dark lens. Gage design also often requires a large pointer movement angle.